Here are a few threads from a while back. Hopefully the info is still valid:
With younger kids, eating is often the biggest hurdle. Making sure he focuses on good food more often than crap, but not trying to eat like a pre-contest bodybuilder. The basics - quality protein, carbs, and healthy fats in each meal, 3 meals a day 7 days a week.
Training-wise, he's young, not underweight, and most likely motivated, so any basic program that has him lifting 4 or 5 days a week should be fine. There are a bunch of solid, reliable routines in the Archives here. Like the food, consistency with the lifting will pay off huge.
Something like this would be a good start: